University of Oxford, Clarendon Laboratory, Oxford OX1 3PU, United Kingdom

In search for correlated quantum states in frustrated magnets we explore the order and excitations in Cs2CuCl4, a quasi-2D spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice, using neutron scattering and high magnetic fields. Unlike conventional magnets with sharp spin-wave excitations, we observe instead dominant excitation continua, as characteristic of fractionalization of spin-1 spin waves into pairs of deconfined spin-1/2 spinons [1,2]. The measured dispersion relations show incommensurate modulations due to the 2D frustrated couplings. The excitation velocity and incommensurate ordering wavevector are largely renormalized from classical values, indicating strong quantum fluctuations in the ground state. In high magnetic fields above saturation spins are fully-polarized in the ground state and the excitations are conventional S=1 spin waves, whose dispersion relation measured by neutron scattering gives the Hamiltonian directly [3].